Waveguides used, or under consideration, for application at frequencies greater than 100 GHz include dominantmode and overmoded rectangular guides, circular low-loss mode guide, microstrip, H and groove guides, various dielectric guide configurations, suspended strip and fin lines, and quasi-optic guides. Most existing systems, such as receivers, operating in the 100–300 GHz range have utilized rectangular TE10guide, or some form of strip line operating in a rectangular guide system. Losses are high, and dimensions are very small, but the system length is short. Neither guide type has the potential for extension into the frequency range much above 300 GHz. A number of other guide types have some encouraging features, such as reasonably low loss, ease of construction or potential for component construction, but none is yet sufficiently advanced to demonstrate practicability to allow systems to give full exploitation to this part of the frequency spectrum.